docs: def validate(self):
          '''Check if ofiles and ifiles match signatures recorded in md5file'''
          if 'TARGET' in env.config['SOS_DEBUG'] or 'ALL' in env.config['SOS_DEBUG']:
              env.log_to_file('TARGET', f'Validating {self.sig_id}')
          #
          # file not exist?
          sig_files = self.input_files._targets + self.output_files._targets + \
              self.dependent_files._targets
          for x in sig_files:
              if not x.target_exists('any'):
                  return f'Missing target {x}'
          #
          sig = request_answer_from_controller(['step_sig', 'get', self.sig_id])
          if not sig:
              return f"No signature found for {self.sig_id}"
          return super(RuntimeInfo, self).validate(sig)